Hot corrosion is an accelerated and often catastrophic surface attack of superalloy gas turbine components often occurring in the temperature range of 600° C. to 1000° C. A superalloy herein is reference to an alloy that typically has a matrix with an austenitic face-centered cubic structure, and whose base alloying elements are nickel, cobalt, nickel-cobalt or nickel-iron. Applications of superalloys which are susceptible to hot corrosion are in the aerospace, industrial gas turbine and marine turbine industries. The above type of accelerated attack is considered to be due primarily to deposits of sodium sulfate (Na2SO4) which when operating at temperatures higher than its melting temperature of 884° C., act as a flux to damage an otherwise protective oxide scale. The Na2SO4 can be ingested in the gas turbine intake air or can be produced by a reaction between sodium chloride (NaCl) in the air and sulfur (S) as an impurity in the fuel. The corrosive effect may be further intensified in marine and other industrial turbines where the alloys may be contaminated with other impurities as well as Na2SO4.
The subject of hot corrosion may be divided into two sub-types: Type I—high-temperature hot corrosion above about 900° C. where pure Na2SO4 is above its melting temperature (884° C.), and Type II—low-temperature hot corrosion (LTHC) between about 600 to 750° C. where a low melting eutectic such as Na2SO4—CoSO4 (melting point 565° C.) is formed on the metal surface. The Type I hot corrosion is characterized by accelerated oxidation and sulfide formation in the alloy matrix. Type II hot corrosion is characterized by pitting corrosion. Prolonged exposure of gas turbine superalloys to low-melting eutectic salts can therefore seriously degrade the durability of the turbine components.
Most conventional protective coatings are based on either alumina (Al2O3) formation or chromia (Cr2O3) formation for high-temperature oxidation and corrosion protection. Theoretical consideration and laboratory tests indicated that both the alumina-forming and chromia-forming coatings are susceptible to Type II hot corrosion. Silica (SiO2)-forming coatings containing a high silicon content have been produced by chemical vapor deposition or pack cementation methods. These coatings are deposited at high substrate temperatures that degrade the mechanical properties of the substrate.